Resistance spot welding method and weld member production method

ABSTRACT

Provided is a resistance spot welding method wherein main current passage includes two or more electrode force application steps including a first electrode force application step and a second electrode force application step following the first electrode force application step, an electrode force F 1  in the first electrode force application step and an electrode force F 2  in the second electrode force application step in the main current passage satisfy a relationship F 1 &gt;F 2 , and an electrode force switching point T f  from the first electrode force application step to the second electrode force application step in the main current passage is set to satisfy predetermined relational formulas.

TECHNICAL FIELD

The present disclosure relates to a resistance spot welding method. Thepresent disclosure is particularly intended to stably ensure a desirednugget diameter without expulsion (splash) regardless of the effect of adisturbance in a sheet combination of three or more overlapping sheetswith a high sheet thickness ratio.

BACKGROUND

Overlapping metal sheets, especially overlapping steel sheets, aretypically joined by resistance spot welding which is one type of lapresistance welding.

Resistance spot welding is a method of squeezing two or more overlappingsteel sheets by a pair of electrodes from above and below and, whileapplying an electrode force, passing a high welding current between theupper and lower electrodes for a short time to join the steel sheets.With this welding method, heat generated from the resistance to the flowof the high welding current is used to form a spot weld. The spot weldis called a nugget. The nugget results from the overlapping steel sheetsmelting and solidifying at their contact portion when the current flowsthrough the steel sheets. The steel sheets are spot-joined by thenugget.

The joining strength of the resistance spot weld depends on the nuggetdiameter. Particularly in the case where high joining strength isrequired as in automotive parts and the like, it is important to ensureat least a certain nugget diameter.

Typically, in the case where the electrode force and the welding time(current passage time) are constant, the nugget diameter graduallyincreases as the welding current increases. When the welding currentreaches a certain value or more, however, expulsion occurs. Expulsion isa phenomenon in which molten metal splatters between the steel sheets.Expulsion causes variations in nugget diameter and joint tensilestrength. This results in unstable weld joint quality.

Regarding the structures of automotive parts, for example, a centerpillar has a structure in which a reinforcement is sandwiched between anouter portion and an inner portion. In this structure, three or moremetal sheets need to be overlapped and spot welded, unlike in the caseof simply spot welding two overlapping metal sheets.

Recent demand to further improve the crashworthiness of automobiles hasencouraged increases in the strength and thickness of reinforcements andthe like. This often creates the need to spot weld a sheet combinationin which an outer portion with small sheet thickness is located on theouter side and an inner portion and a reinforcement each with largesheet thickness are located on the inner side.

Of the metal sheets in the sheet combination, a metal sheet withrelatively small sheet thickness is referred to as a thin sheet, and ametal sheet with relatively large sheet thickness as a thick sheet. Thesame applies hereafter.

In the case where such a sheet combination of three or more overlappingsheets with a high sheet thickness ratio ((the total thickness of thesheet combination)/(the sheet thickness of the thinnest metal sheet inthe sheet combination)) is subjected to conventional resistance spotwelding using a constant electrode force and welding current, a nuggetof a required size is hard to be formed between the outermost thin sheet(i.e. sheet in contact with the electrode tip) and the thick sheet. Thistendency is particularly noticeable when the sheet thickness ratio ofthe sheet combination is more than 3, and further noticeable when thesheet thickness ratio is 5 or more.

This is considered to be because the contact with the electrode tiphinders the increase of the temperature between the outermost thin sheetand the thick sheet.

In detail, the nugget is typically formed by heat generated by volumeresistance according to the specific resistance of each steel sheet fromnear the center between the electrodes. However, the contact with theelectrode tip hinders the increase of the temperature between theoutermost thin sheet and the thick sheet. Accordingly, the nugget growslarge between the thick sheets located near the center between theelectrodes before it grows between the thin sheet and the thick sheet.Consequently, molten metal cannot be suppressed by the applied electrodeforce, and expulsion occurs.

Since the outer portion is required to have formability, mild steel isoften used as the thin sheet used in the outer portion. Meanwhile, theinner portion and the reinforcement are strength reinforcing members,and therefore high tensile strength steel sheets are often used as thethick sheets used in these members. In such a sheet combination of thinand thick sheets, the heat generation position tends to be closer to thehigh tensile strength steel sheet (thick sheet) side with high specificresistance. Moreover, in the case where the metal sheets used are coatedsteel sheets, coated layers melted at low temperature cause the currentpath between the steel sheets to expand and the current density todecrease. This further hinders the formation of the nugget between thethin sheet and the thick sheet.

As a resistance spot welding method used for such a sheet combination ofthree or more overlapping sheets with a high sheet thickness ratio, forexample, WO 2014/045431 A1 (PTL 1) proposes “A spot welding method forhigh strength steel sheets with excellent joint strength, comprisingresistance spot welding a plurality of steel sheets overlapped on eachother, wherein the plurality of steel sheets are either two steel sheetsthat each have a tensile strength of 780 MPa or more and 1850 MPa orless and that have a sheet thickness ratio (={the sum of the sheetthicknesses of the steel sheets}/{the sheet thickness of a thinner steelsheet (in the case where the steel sheets have the same sheet thickness,the sheet thickness of one sheet)}) of 2 or more and 5 or less or threesteel sheets that are composed of three steel sheets each having atensile strength of 780 MPa or more and 1850 MPa or less or composed oftwo steel sheets each having a tensile strength of 780 MPa or more and1850 MPa or less and one steel sheet located on an outer side of the twosteel sheets and having a tensile strength of less than 780 MPa and thathave a sheet thickness ratio (={the sum of the sheet thicknesses of thesteel sheets}/{the sheet thickness of a thinner steel sheet (in the casewhere the steel sheets have the same sheet thickness, the sheetthickness of one sheet)}) of 3 or more and 6 or less, the spot weldingincludes: a first current passage step that is preliminary currentpassage with an electrode force P1 (kN) and a welding current I1 (kA);and a second current passage step that is main current passage with anelectrode force P2 (kN) and a welding current I2 (kA), the electrodeforces P1 and P2 are within a range defined by the following formulas(2) and (3):

0.5≤P2≤3.0t ^((1/3))  (2)

1.0×P2<P1≤2.0×P2  (3)

where t (mm) is an average sheet thickness of the plurality of steelsheets, the welding current I1 is 30% or more and 90% or less of thewelding current I2, and the second current passage step starts within0.1 (s) after the first current passage step ends.

CITATION LIST Patent Literature

PTL 1: WO 2014/045431 A1

SUMMARY Technical Problem

In the case where there is a disturbance during welding, such as when apoint that has already been welded (hereafter referred to as “existingweld”) is present near the current welding point or when the parts to bewelded have considerable surface roughness and a contact point of theparts to be welded is present near the welding point, part of thecurrent is shunted into such existing weld or contact point duringwelding. In this state, even when current passage is performed under apredetermined condition, the current density at the position to bewelded which is directly above and below the electrodes decreases, andconsequently a nugget of a required diameter cannot be obtained.

Moreover, in the case where the surroundings of the welding point arestrongly restrained due to surface roughness, member shape, etc., thesheet gap between the steel sheets increases. This causes a smallercontact diameter of the steel sheets, as a result of which a nugget of arequired diameter cannot be obtained or expulsion is facilitated.

With the technique described in PTL 1, in the case where there is a gapbetween the steel sheets as the parts to be welded, the electrode forcein the first step is set to be greater than the electrode force in thesecond step in order to secure a sufficient contact area between thesteel sheets in the first step in the initial stage of current passage.

However, in real operation such as vehicle manufacturing, parts to bewelded conveyed one after another are welded continuously. Thedisturbance state varies among welding positions or parts to be welded,due to the work conditions, the dimensional errors of the parts to bewelded, or the like. It is therefore difficult to accurately grasp thedisturbance state of the parts to be welded before actually startingwelding.

Thus, the technique described in PTL 1 has a problem in that a desirednugget diameter cannot be ensured without expulsion when a greaterdisturbance than expected occurs.

As mentioned above, the disturbance state varies among welding positionsor parts to be welded, due to the work conditions, the dimensionalerrors of the parts to be welded, or the like. Hence, even if thedisturbance state of the parts to be welded can be grasped beforewelding, it is necessary to set, for each disturbance state, an optimalwelding condition based on the disturbance state, which is problematicin terms of work efficiency and costs.

It could therefore be helpful to provide a resistance spot weldingmethod that can stably ensure a desired nugget diameter withoutexpulsion regardless of the effect of a disturbance particularly in asheet combination of three or more overlapping sheets with a high sheetthickness ratio.

It could also be helpful to provide a weld member production method ofjoining a plurality of overlapping metal sheets by the resistance spotwelding method.

Solution to Problem

We conducted intensive study to achieve the object stated above, anddiscovered the following:

(1) In the case where main current passage for forming a nugget isdivided into two or more electrode force application steps and theelectrode force in the first electrode force application step is greaterthan the electrode force in the second electrode force application step,whether a desired current path is ensured, that is, whether a desiredheat generation pattern is obtained, can be determined from the timeintegration value of the resistance between the electrodes from thecurrent passage start of the main current passage to when apredetermined time elapses, regardless of the effect of a disturbance.

(2) Moreover, the effect of the disturbance can be mitigated effectivelyby setting, depending on the time integration value of the resistancebetween the electrodes, the timing of switching from the electrode forcein the first electrode force application step to the electrode force inthe second electrode force application step.

(3) It is therefore possible to stably ensure a desired nugget diameterwithout expulsion by effectively responding to variations in thedisturbance state, even when continuously welding parts to be weldedwhich are conveyed one after another in real operation such as vehiclemanufacturing (i.e. even when the disturbance state varies among weldingpositions or parts to be welded).

The present disclosure is based on these discoveries and furtherstudies.

We thus provide:

1. A resistance spot welding method of squeezing, by a pair ofelectrodes, parts to be welded which are a plurality of overlappingmetal sheets and passing a current while applying an electrode force tojoin the parts to be welded, wherein main current passage includes twoor more electrode force application steps including a first electrodeforce application step and a second electrode force application stepfollowing the first electrode force application step, and an electrodeforce F₁ in the first electrode force application step and an electrodeforce F₂ in the second electrode force application step in the maincurrent passage satisfy a relationship F₁>F_(2,) and an electrode forceswitching point T_(f) from the first electrode force application step tothe second electrode force application step in the main current passageis set to satisfy the following Formulas (1) to (3):

in the case where T_(A)≤0.8 ×T₀,

T _(A) ≤T _(f) <T ₀  (1)

in the case where 0.8×T ₀ <T _(A) ≤T ₀ or 0.9×R ₀ ≤R _(A) ≤R ₀,

0.9×T ₀ <T _(f)<1.1×T ₀  (2)

in the case where R_(A<)0.9×R₀,

T ₀ <T _(f) T ₀+2×(R ₀ −R _(A))/R ₀ ×T _(m)  (3)

where T₀ is a reference electrode force switching point from the firstelectrode force application step to the second electrode forceapplication step, T_(m) is a total welding time in the main currentpassage, R_(A) is a time integration value of a resistance between theelectrodes from current passage start of the main current passage to thereference electrode force switching point T₀, R₀ is a time integrationvalue of a resistance between the electrodes from current passage startto the reference electrode force switching point T₀ in the case wherecurrent passage is performed under a same condition as the main currentpassage when the parts to be welded have no disturbance, and T_(A) is atime at which a time integration value of a resistance between theelectrodes in the main current passage reaches R₀.

2. The resistance spot welding method according to 1., wherein thereference electrode force switching point T₀ satisfies the followingformula:

0.1×T _(m) ≤T ₀≤0.8×T _(m).

3. The resistance spot welding method according to 1. or 2., comprising:performing test welding; and performing actual welding including themain current passage, after the test welding, wherein in main currentpassage in the test welding, a time variation curve of an instantaneousamount of heat generated per unit volume and a cumulative amount of heatgenerated per unit volume that are calculated from an electricalproperty between the electrodes in forming an appropriate nugget byperforming current passage by constant current control are stored, andin the main current passage in the actual welding, the time variationcurve of the instantaneous amount of heat generated per unit volume andthe cumulative amount of heat generated per unit volume that are storedin the main current passage in the test welding are set as a target, anda current passage amount is controlled according to the target.

4. A weld member production method comprising joining a plurality ofoverlapping metal sheets by the resistance spot welding method accordingto any one of 1. to 3.

Advantageous Effect

It is thus possible to stably ensure a desired nugget diameter withoutexpulsion regardless of the effect of a disturbance even in a sheetcombination of three or more overlapping sheets with a high sheetthickness ratio.

It is also possible to stably ensure a desired nugget diameter byeffectively responding to variations in the disturbance state, even whencontinuously welding parts to be welded which are conveyed one afteranother in real operation such as vehicle manufacturing (i.e. even whenthe disturbance state varies among welding positions or parts to bewelded). This is very advantageous in improving the operating efficiencyand the yield rate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram illustrating the relationship between the electrodeforce and time in main current passage and the relationship between thetime integration value of the resistance between the electrodes and timein the main current passage when Formula (1) is satisfied (in the casewhere T_(A)≤0.8×T₀) in a resistance spot welding method according to oneof the disclosed embodiments;

FIG. 2 is a diagram illustrating the relationship between the electrodeforce and time in the main current passage and the relationship betweenthe time integration value of the resistance between the electrodes andtime in the main current passage when Formula (2) is satisfied (in thecase where 0.8×T₀<T_(A)≤T₀ or 0.9×R₀≤R_(A)≤R₀) in the resistance spotwelding method according to one of the disclosed embodiments;

FIG. 3 is a diagram illustrating the relationship between the electrodeforce and time in the main current passage and the relationship betweenthe time integration value of the resistance between the electrodes andtime in the main current passage when Formula (3) is satisfied (in thecase where R_(A)<0.9×R₀) in the resistance spot welding method accordingto one of the disclosed embodiments;

FIG. 4A is a diagram schematically illustrating an example of performingwelding in a state of no disturbance;

FIG. 4B is a diagram schematically illustrating an example of performingwelding in a state of no disturbance;

FIG. 5A is a diagram schematically illustrating an example of performingwelding on a sheet combination having a sheet gap;

FIG. 5B is a diagram schematically illustrating an example of performingwelding on a sheet combination having a sheet gap;

FIG. 6A is a diagram schematically illustrating an example of performingwelding on a sheet combination having existing welds; and

FIG. 6B is a diagram schematically illustrating an example of performingwelding on a sheet combination having existing welds.

DETAILED DESCRIPTION

One of the disclosed embodiments will be described below.

One of the disclosed embodiments is a resistance spot welding method ofsqueezing, by a pair of electrodes, parts to be welded which are aplurality of overlapping metal sheets and passing a current whileapplying an electrode force to join the parts to be welded, wherein maincurrent passage includes two or more electrode force application stepsincluding a first electrode force application step and a secondelectrode force application step following the first electrode forceapplication step, an electrode force F₁ in the first electrode forceapplication step and an electrode force F₂ in the second electrode forceapplication step in the main current passage satisfy a relationshipF₁>F₂, and an electrode force switching point T_(f) from the firstelectrode force application step to the second electrode forceapplication step in the main current passage (hereafter also referred toas “electrode force switching point T_(f)”) is set to satisfy apredetermined relationship.

The electrode force switching point T_(f) (and the below-describedreference electrode force switching point T₀) is the time at which theelectrode force switching operation starts.

The electrode force switching point T_(f) (and the below-describedreference electrode force switching point T₀) is expressed relative tothe current passage start point of the main current passage (i.e.expressed as the time elapsed from the current passage start point ofthe main current passage).

The same applies to the below-described T_(A) (the time at which thetime integration value of the resistance between the electrodes in themain current passage reaches R₀), etc.

The resistance spot welding method according to one of the disclosedembodiments is particularly suitable for a sheet combination whose sheetthickness ratio ((the total thickness of the sheet combination)/(thesheet thickness of the thinnest metal sheet in the sheet combination))is more than 3 and further suitable for a sheet combination whose sheetthickness ratio is 5 or more, for which it has been difficult to obtaina nugget of a required size between thin and thick sheets withoutexpulsion regardless of a disturbance.

The resistance spot welding method is effective for a sheet combinationof two overlapping sheets as well.

The term “thin sheet” means a metal sheet with relatively small sheetthickness and the term “thick sheet” means a metal sheet with relativelylarge sheet thickness, of the steel sheets used in the sheetcombination. The sheet thickness of a thin sheet is not greater than ¾of that of a metal sheet (thick sheet) with the largest sheet thickness.

Any welding device that includes a pair of upper and lower electrodesand is capable of freely controlling each of the electrode force and thewelding current during welding may be used in the resistance spotwelding method according to one of the disclosed embodiments. The type(stationary, robot gun, etc.), the electrode shape, and the like are notlimited.

The resistance spot welding method according to one of the disclosedembodiments will be described below.

(A) Main current passage (also referred to as “main current passage inactual welding” in order to be distinguished from main current passagein test welding (described later). The term “main current passage” whenused alone denotes the main current passage in the actual welding andnot the main current passage in the test welding. Herein, the “maincurrent passage” denotes current passage for forming a nugget. The“actual welding” denotes a process of actually welding parts to bewelded, which is to be distinguished from the test welding.)

For a sheet combination of three or more overlapping sheets with a highsheet thickness ratio, by dividing the main current passage for nuggetformation into two or more electrode force application steps andsatisfying the following relationship, i.e. setting the electrode forceF₁ in the first electrode force application step (hereafter also simplyreferred to as “F₁”) to be greater than the electrode force in thesecond electrode force application step (hereafter also simply referredto as “F₂”), the contact diameter between the metal sheets as the partsto be welded can be ensured more advantageously as mentioned earlier:

F ₁ >F ₂.

Preferably, F₁≥1.1×F₂. More preferably, F₁≥1.2×F₂. Further preferably,F₁≥1.5×F₂.

F₁ and F₂ may be set as appropriate depending on the materials,thicknesses, etc. of the metal sheets as the parts to be welded, as longas the foregoing relationship is satisfied.

For example, in the case of using a sheet combination of three or moreoverlapping sheets with a high sheet thickness ratio (e.g. a sheetcombination of three overlapping sheets composed of two thick sheets(mild steel or 490 MPa to 2000 MPa-grade zinc or zinc alloy coated steelsheets or non-coated steel sheets of 0.8 mm to 3.0 mm in thickness) andone thin sheet (zinc or zinc alloy coated steel sheet or non-coatedsteel sheet (mild steel) of 0.5 mm to 2.0 mm in thickness), it ispreferable that F₁ is 2.0 kN to 10.0 kN and F₂ is 1.0 kN to 6.0 kN.

n the case of using a typical sheet combination of two overlappingsheets, it is preferable that F₁ is 2.0 kN to 7.0 kN and F₂ is 1.0 kN to5.0 kN.

In the resistance spot welding method according to one of the disclosedembodiments, it is important to set the timing of switching from F₁ toF_(2,) i.e. the electrode force switching point T_(f), so as to satisfythe following Formulas (1) to (3), depending on the time integrationvalue of the resistance between the electrodes from the current passagestart of the main current passage to when a predetermined time elapses:

in the case where T_(A)≤0.8×T₀,

T _(A) ≤T _(f) <T ₀  (1)

in the case where 0.8×T₀<T_(A)≤T₀or 0.9×R₀≤R_(A)≤R₀,

0.9×T ₀ <T _(f)<1.1×T ₀  (2)

in the case where R_(A)<0.9×R₀,

T ₀ <T _(f) ≤T ₀+2×(R ₀ −R _(A))/R ₀ ×T _(m)  (3)

where T₀ is the reference electrode force switching point from the firstelectrode force application step to the second electrode forceapplication step, T_(m) is the total welding time in the main currentpassage, R_(A) is the time integration value of the resistance betweenthe electrodes from the current passage start of the main currentpassage to the reference electrode force switching point T₀, R₀ is thetime integration value of the resistance between the electrodes from thecurrent passage start to the reference electrode force switching pointT₀ in the case where current passage is performed under the samecondition as the main current passage when the parts to be welded haveno disturbance, and T_(A) is the time at which the time integrationvalue of the resistance between the electrodes in the main currentpassage reaches R₀.

In the case where T_(A)≤0.8×T₀, that is, in the case where R_(A) isexpected to be greater than R₀ by a certain amount (see FIG. 1 ), forexample when subjecting sheet combinations each having a zinc coatedsteel sheet on the outer side to continuous welding (hereafter alsoreferred to as “continuous spot welding”) using the same electrodes, theelectrodes are alloyed with zinc and the surface resistance increases asthe number of welding operations increases. In other words, ascontinuous spot welding progresses, the resistance between theelectrodes in each welding operation increases. Actually, however, anincrease in the contact area between the electrode and the metal sheetcauses the current density to decrease, so that the nugget diametertends to decrease. In such a case, it is effective to advance the timingof switching from F₁ to F₂. Specifically, it is effective to set theelectrode force switching point T_(f) so as to satisfy the foregoingFormula (1).

In the case where R_(A)<0.9×R₀, that is, in the case where R_(A) is lessthan R₀ by a certain amount (see FIG. 3 ), for example when the metalsheets warp due to a sheet gap (i.e. an electrode force is applied to asheet combination having a sheet gap by the electrodes and as a resultthe metal sheets warp due to the sheet gap), the contact area betweenthe electrode and the metal sheet increases and the resistance betweenthe electrodes decreases. However, the contact area between the metalsheets is not sufficient. Switching from F₁ to F₂ in this state maycause expulsion. In such a case, it is effective to delay the timing ofswitching from F₁ to F_(2.) Specifically, it is effective to set theelectrode force switching point T_(f) so as to satisfy the foregoingFormula (3), to suppress expulsion. This is further effective whencurrent increases in the below-described adaptive control welding or thelike.

In the case where 0.8×T₀<T_(A)≤T₀ or 0.9 ×R₀≤R_(A)≤R₀ (see FIG. 2 ),that is, in the case where R_(A) is (or is expected to be) approximatelyequal to R₀, the effect of a disturbance is unlikely to be significant.In such a case, the electrode force switching point T_(f) is set so asto satisfy the foregoing Formula (2).

Thus, in the resistance spot welding method according to one of thedisclosed embodiments, it is important to set the electrode forceswitching point T_(f) so as to satisfy Formulas (1) to (3) depending on,for example, the time integration value of the resistance between theelectrodes from the current passage start of the main current passage towhen the predetermined time

Formulas (1) to (3) are more preferably the following Formulas (1)′ to(3)′ respectively:

in the case where T_(A)0.8×T₀,

T _(A) ≤T _(f)≤0.95×T ₀  (1)′

in the case where 0.8×T₀<T_(A)≤T₀or 0.9×R₀≤R_(A)≤R₀,

0.95×T ₀ <T _(f)<1.05×T ₀  (2)′

in the case where R_(A)<0.9×R₀,

1.05×T ₀ ≤T _(f) ≤T ₀+2×(R ₀ −R _(A))/R ₀ ×T _(m)  (3)'.

For example, the time integration value R₀ of the resistance between theelectrodes from the current passage start to the reference electrodeforce switching point To in the case where current passage is performedunder the same condition as the main current passage when the parts tobe welded have no disturbance may be obtained by separately preparingparts to be welded composed of metal sheets of the same sheetthicknesses and materials as in the main current passage and having nodisturbance and conducting a preliminary welding test of welding theparts to be welded under the same condition as the main current passage.

In the case of performing the below-described test welding, the timeintegration value of the resistance between the electrodes from thecurrent passage start to the reference electrode force switching pointT₀ in the main current passage in the test welding may be R₀.

The reference electrode force switching point T₀ (ms) from the firstelectrode force application step to the second electrode forceapplication step may be set as appropriate depending on, for example,the materials and thicknesses of the metal sheets as the parts to bewelded, but is preferably set using the total welding time T_(m) (ms) inthe main current passage so as to satisfy the following formula:

0.1×T _(m) ≤T ₀0.8×T _(m).

If T₀ is less than 0.1×T_(m), there is a possibility that the effect ofa disturbance cannot be mitigated effectively by controlling theelectrode force switching timing. If T₀ is more than 0.8×T_(m), too,there is a possibility that the effect of a disturbance cannot bemitigated effectively by controlling the electrode force switchingtiming. T₀ is therefore preferably 0.1×T_(m) or more and 0.8×T_(m) orless.

T₀ is more preferably 0.2×T_(m) or more, and further preferably0.3×T_(m) or more. T₀ is more preferably 0.75×T_(m) or less, and furtherpreferably 0.7×T_(m) or less.

The total welding time T_(m) (ms) in the main current passage may be setas appropriate depending on, for example, the materials and thicknessesof the metal sheets as the parts to be welded.

For example, in the case of using a sheet combination of three or moreoverlapping sheets with a high sheet thickness ratio as mentioned above,T_(m) is preferably 120 ms to 1000 ms. In the case of using a typicalsheet combination of two overlapping sheets, T_(m) is preferably 80 msto 800 ms.

In the case where the main current passage is divided into two or morecurrent passage steps and a cooling time is provided between the currentpassage steps, the total welding time in the main current passageincludes the cooling time between the current passage steps.

The main current passage may be performed by constant current control.Alternatively, after performing the below-described test welding,adaptive control welding of controlling the current passage amountaccording to the target set in the test welding may be performed.

In the case of constant current control, the welding current may be setas appropriate depending on, for example, the materials and thicknessesof the metal sheets as the parts to be welded. The main current passagemay be divided into two or more current passage steps, and a coolingtime may be provided between the current passage steps.

The timing of dividing the current passage may be the same as ordifferent from the timing of dividing the electrode force application.The point of switching the current value from the first current passagestep to the second current passage step (i.e. the timing of dividing thecurrent passage) in the main current passage need not be changedaccording to the change of the electrode force switching point in themain current passage. The same applies to the below-described adaptivecontrol welding.

For example, in the case of welding a typical sheet combination of twooverlapping sheets by one current passage step, the current value ispreferably 4.0 kA to 12.0 kA.

In the case of performing welding by two or more current passage stepsobtained by dividing the current passage, it is preferable that thecurrent value and the welding time in the first current passage step are3.0 kA to 12.0 kA and 40 ms to 800 ms respectively and the current valueand the welding time in the second current passage step are 4.0 kA to14.0 kA and 20 ms to 400 ms respectively. Particularly in the case ofwelding a sheet combination of three or more overlapping sheets with ahigh sheet thickness ratio as mentioned above, it is preferable that thecurrent value in the first current passage step is less than the currentvalue in the second current passage step. In the case where a coolingtime is provided between the first current passage step and the secondcurrent passage step, the cooling time is preferably 10 ms to 400 ms.

In the case of adaptive control welding, welding is performed accordingto the target (the time variation curve of the instantaneous amount ofheat generated per unit volume and the cumulative amount of heatgenerated) obtained as a result of the below-described test welding. Ifthe amount of time variation of the instantaneous amount of heatgenerated per unit volume follows the time variation curve, the weldingis continued without change and completed. If the amount of timevariation of the instantaneous amount of heat generated per unit volumediffers from the time variation curve, the current passage amount iscontrolled in order to compensate for the difference within a remainingwelding time so that the cumulative amount of heat generated per unitvolume in the actual welding matches the cumulative amount of heatgenerated per unit volume set as the target.

In the case of adaptive control welding, too, the main current passagemay be divided into two or more current passage steps, and adaptivecontrol welding may be performed for each current passage step.

In detail, the main current passage in the actual welding and the maincurrent passage in the test welding are each divided into two or morecurrent passage steps so as to correspond to each other.

Welding is then performed according to the target (the time variationcurve of the instantaneous amount of heat generated per unit volume andthe cumulative amount of heat generated) for each current passage stepobtained as a result of the test welding. If the amount of timevariation of the instantaneous amount of heat generated per unit volumediffers from the time variation curve in any current passage step, thecurrent passage amount is controlled in order to compensate for thedifference within a remaining welding time in the current passage stepso that the cumulative amount of heat generated per unit volume in thecurrent passage step matches the cumulative amount of heat generated perunit volume in the current passage step in the test welding.

The method of calculating the amount of heat generated is not limited.JP H11-33743 A discloses an example of the method, which may be usedherein. The following is the procedure of calculating the amount q ofheat generated per unit volume and per unit time and the cumulativeamount Q of heat generated per unit volume according to this method.

Let t be the total thickness of the parts to be welded, r be theelectrical resistivity of the parts to be welded, V be the voltagebetween the electrodes, I be the welding current, and S be the contactarea of the electrodes and the parts to be welded. In this case, thewelding current passes through a columnar portion whose cross-sectionalarea is S and thickness is t, to generate heat by resistance. The amountq of heat generated per unit volume and per unit time in the columnarportion is given by the following Formula (4):

q=(V·I)/(S·t)  (4).

The electrical resistance R of the columnar portion is given by thefollowing Formula (5):

R=(r·t)/S  (5).

Solving Formula (5) for S and substituting the solution into Formula (4)yields the amount q of heat generated as indicated by the followingFormula (6):

q=(V·I·R)/(r·t ²)=(V ²)/(r·t ²)  (6).

As is clear from Formula (6), the amount q of heat generated per unitvolume and per unit time can be calculated from the voltage V betweenthe electrodes, the total thickness t of the parts to be welded, and theelectrical resistivity r of the parts to be welded, and is not affectedby the contact area S of the electrodes and the parts to be welded.Although the amount of heat generated is calculated from the voltage Vbetween the electrodes in Formula (6), the amount q of heat generatedmay be calculated from the interelectrode current I. The contact area Sof the electrodes and the parts to be welded need not be used in thiscase, either. By cumulating the amount q of heat generated per unitvolume and per unit time for the welding time, the cumulative amount Qof heat generated per unit volume for the welding is obtained. As isclear from Formula (6), the cumulative amount Q of heat generated perunit volume can also be calculated without using the contact area S ofthe electrodes and the parts to be welded.

Although the above describes the case of calculating the cumulativeamount Q of heat generated by the method described in JP H11-33743 A,the cumulative amount Q may be calculated by any other method.

(B) Test Welding

In the case of performing the main current passage in the actual weldingby adaptive control welding, the test welding is performed before theactual welding. In the main current passage in the test welding, thetime variation curve of the instantaneous amount of heat generated perunit volume and the cumulative amount of heat generated per unit volumethat are calculated from the electrical property between the electrodesin forming an appropriate nugget by performing current passage byconstant current control are stored.

In detail, in the test welding, a preliminary welding test with the samesteel types and thicknesses as the parts to be welded in the actualwelding is performed with various conditions by constant current controlin a state without a sheet gap or current shunting to an existing weld,to find an optimal condition in the test welding.

Current passage is then performed under this condition, and the timevariation curve of the instantaneous amount of heat generated per unitvolume and the cumulative amount of heat generated per unit volume thatare calculated from the electrical property between the electrodesduring the current passage are stored as the target in the actualwelding. Herein, the “electrical property between the electrodes” meansthe resistance between the electrodes or the voltage between theelectrodes.

The main current passage in the test welding may be divided into two ormore current passage steps, and adaptive control welding may beperformed for each current passage step in the actual welding, asmentioned above.

In the case of welding a sheet combination of three or more overlappingsheets with a high sheet thickness ratio as mentioned above, it ispreferable that the current value in the first current passage step isless than the current value in the second current passage step in thetest welding, too.

(C) Other Modifications Preliminary current passage for stabilizing thecontact diameter may be performed before the main current passage (themain current passage in the actual welding and/or the test welding) fornugget formation, and subsequent current passage for subsequent heattreatment may be performed. The preliminary current passage and thesubsequent current passage may be performed by constant current control,or performed in an upslope or downslope current pattern.

A cooling time may be provided between the preliminary current passageand the main current passage and between the main current passage andthe subsequent current passage.

The parts to be welded are not limited. The resistance spot weldingmethod may be used for welding of steel sheets and coated steel sheetshaving various strengths from mild steel to ultra high tensile strengthsteel and light metal sheets of aluminum alloys and the like. Theresistance spot welding method may also be used for a sheet combinationof three or more overlapping steel sheets.

By joining a plurality of overlapping metal sheets by the resistancespot welding method described above, various weld members, in particularweld members of automotive parts and the like, can be produced whilestably ensuring a desired nugget diameter by effectively responding tovariations in the disturbance state.

EXAMPLES

The presently disclosed techniques will be described below by way ofexamples. The conditions in the examples are one example of conditionsemployed to determine the operability and effects of the presentlydisclosed techniques, and the present disclosure is not limited to suchexample of conditions. Various conditions can be used in the presentdisclosure as long as the object of the present disclosure is fulfilled,without departing from the scope of the present disclosure.

Actual welding (main current passage) was performed for each sheetcombination of metal sheets listed in Table 1 under the conditionslisted in

Tables 3 and 4 in the states illustrated in FIGS. 4A to 6B, to produceweld joints.

In FIGS. 5A and 5B, spacers 15 were inserted between metal sheets, and asheet combination was clamped from above and below (not illustrated) tocreate a sheet gap of any of various thicknesses. The distance betweenthe spacers was 60 mm in each case.

In FIGS. 6A and 6B, there were two existing welds 16, and the weldingposition (the center between the electrodes) was adjusted to be amidpoint between the existing welds (i.e. the distance L from eachexisting weld was equal). The nugget diameter of each existing weld was4√t′ (mm) (where t′ is the sheet thickness (mm) of the thinnest metalsheet in the sheet combination).

In No. 1-5, to simulate the alloying state of the electrodes and zinc(which occurs when subjecting a sheet combination having a zinc coatedsteel sheet on the outer side to continuous spot welding), a separatelyprepared sheet combination having a zinc coated steel sheet on the outerside was resistance spot welded at 1000 points. After this, theelectrodes used in the resistance spot welding at 1000 points was usedto perform actual welding.

In some examples, before actual welding, test welding was performedunder the conditions listed in Table 2 in a state of no disturbanceillustrated in FIGS. 4A and 4B, and the time variation curve of theinstantaneous amount of heat generated per unit volume and thecumulative amount of heat generated per unit volume in the main currentpassage in the test welding were stored. Moreover, the time integrationvalue of the resistance between the electrodes from the current passagestart to the reference electrode force switching point T₀ in the maincurrent passage in the test welding was measured and taken to be R₀.

In each example in which current passage was performed by constantcurrent control, parts to be welded composed of metal sheets of the samesheet thicknesses and materials as in the main current passage andhaving no disturbance were separately prepared, and a preliminarywelding test of welding the parts to be welded under the same conditionas the actual welding was performed to obtain R₀.

R₀ thus obtained is listed in Table 3.

For each produced weld joint, the weld was cut and etched in section,and then observed with an optical microscope to evaluate the weld jointin the following three levels A, B, and F based on the nugget diameterand whether expulsion occurred. For a sheet combination of threeoverlapping sheets, the evaluation was made using the diameter of anugget formed between the thinnest metal sheet 11 on the outer side andthe metal sheet 12 adjacent to the metal sheet 11. The evaluationresults are listed in Table 4.

A: The nugget diameter was 4.5√t′ (mm) or more (where t′ is the sheetthickness (mm) of the thinnest metal sheet in the sheet combination) andno expulsion occurred regardless of a disturbance.

B: The nugget diameter was 4√t′ (mm) or more and no expulsion occurredregardless of a disturbance (excluding the case A).

F: The nugget diameter was less than 4√t′ (mm) and/or expulsion occurreddepending on a disturbance.

TABLE 1 Sheet combination Metal sheet of reference sign 11 in Metalsheet of reference sign 12 in the Metal sheet of reference sign 13 in4√t′ 4.5√t′ ID the drawings drawings the drawings (mm) (mm) A 270MPa-grade GA steel sheet 1470 MPa-grade GA steel sheet 1470 MPa-grade GAsteel sheet 3.35 3.76 (sheet thickness: 0.7 mm) (sheet thickness: 1.6mm) (sheet thickness: 1.6 mm) B 270 MPa-grade GA steel sheet 1470MPa-grade cold-rolled steel sheet 1470 MPa-grade GA steel sheet 3.584.02 (sheet thickness: 0.8 mm) (sheet thickness: 1.6 mm) (sheetthickness: 1.4 mm) C 1470 MPa-grade GA steel sheet 1470 MPa-grade GAsteel sheet 4.73 5.32 (sheet thickness: 1.4 mm) (sheet thickness: 1.4mm)

TABLE 2 Test welding condition Electrode force Current passage conditionapplication condition First current Second current Switching passagestep passage step Sheet point from Current Welding Cooling CurrentWelding combination F₁′*1 F₂′*2 F₁′ to F₂′ value time time value timeNo. ID (kN) (kN) (ms) (kA) (ms) (ms) (kA) (ms) Remarks 1 1-1 A 5.0 3.0160 5.0 160 40 6.5 200 Example 1-2 1-3 1-4 1-5 2 2-1 B — Example 2-2 2-33 3-1 C 4.5 3.0 200 7.0 200 — 7.0 120 Example 3-2 3-3 3-4 4 4-1 A —Comparative 4-2 Example 4-3 4-4 *1: Electrode force in first electrodeforce application step in main current passage in test welding *2:Electrode force in second electrode force application step in maincurrent passage in test welding

TABLE 3 Actual welding condition Electrode force switching Electrodeforce Sheet timing application condition combination change F₁ F₂ T_(f)T₀ T_(m) No. ID Disturbance state control (kN) (kN) (ms) (ms) (ms) 1 1-1A None Applied 5.0 3.0 162 160 400 1-2 Sheet gap (tg = 0.5 mm) 210 1-3Sheet gap (tg = 1 mm) 280 1-4 Existing weld (L = 10 mm) 320 1-5 Alloyingof electrodes and Zn 142 2 2-1 B None Applied 5.5 3.0 145 140 380 2-2Sheet gap (tg = 0.5 mm) 220 2-3 Sheet gap (tg = 1 mm) 260 3 3-1 C NoneApplied 4.5 3.0 200 200 320 3-2 Sheet gap (tg = 0.5 mm) 260 3-3 Sheetgap (tg = 1 mm) 280 3-4 Existing weld (L = 10 mm) 270 4 4-1 A None Notapplied 5.0 2.5 160 160 400 4-2 Sheet gap (tg = 0.5 mm) 160 4-3 Sheetgap (tg = 1 mm) 160 4-4 Existing weld (L = 10 mm) 160 Actual weldingcondition R₀ R_(A) Appropriate range of T_(A) (Ω · (Ω · T_(f) accordingto T₀/T_(m) (ms) ms) ms) R_(A)/R₀ Formulas (1) to (3) Remarks 1 1-1 0.40158 0.041 0.042 1.02 More than 144 to less Formula (2) Example than 1761-2 200 0.036 0.88 More than 160 to 258 Formula (3) 1-3 270 0.032 0.78More than 160 to 336 Formula (3) 1-4 290 0.030 0.73 More than 160 to 375Formula (3) 1-5 125 0.050 1.22 125 to less than 160 Formula (1) 2 2-10.37 142 0.038 0.036 0.95 More than 126 to less Formula (2) Example than154 2-2 210 0.033 0.87 More than 140 to 240 Formula (3) 2-3 240 0.0280.74 More than 140 to 340 Formula (3) 3 3-1 0.63 190 0.044 0.046 1.05More than 180 to less Formula (2) Example than 220 3-2 258 0.039 0.89More than 200 to 273 Formula (3) 3-3 268 0.037 0.84 More than 200 to 302Formula (3) 3-4 260 0.038 0.86 More than 200 to 287 Formula (3) 4 4-10.40 156 0.041 0.043 1.05 More than 144 to less Formula (2) Comparativethan 176 Example 4-2 210 0.035 0.85 More than 160 to 277 Formula (3) 4-3290 0.031 0.76 More than 160 to 355 Formula (3) 4-4 310 0.029 0.71 Morethan 160 to 394 Formula (3)

TABLE 4 Actual welding condition Current passage condition First currentSecond current passage step passage step Evaluation result Sheet CurrentWelding Cooling Current Welding Nugget combination Current passage valuetime time value time diameter No. ID method (kA) (ms) (ms) (kA) (ms)(mm) Expulsion Evaluation Remarks 1 1-1 A Adaptive control — 160 40 —200 4.0 Not occurred A Example 1-2 4.1 Not occurred 1-3 3.9 Not occurred1-4 4.1 Not occurred 1-5 4.2 Not occurred 2 2-1 B Constant current 5.5140 60 7.0 180 4.3 Not occurred B Example 2-2 control 4.2 Not occurred2-3 3.6 Not occurred 3 3-1 C Adaptive control — 200 — — 120 5.4 Notoccurred A Example 3-2 5.4 Not occurred 3-3 5.6 Not occurred 3-4 5.5 Notoccurred 4 4-1 A Constant current 5.0 160 40 6.5 200 4.0 Not occurred FComparative 4-2 control 3.3 Not occurred Example 4-3 3.2 Occurred 4-42.9 Not occurred

In each Example, a sufficient nugget diameter was obtained withoutexpulsion regardless of a disturbance.

In each Comparative Example, a sufficient nugget diameter was notobtained and/or expulsion occurred depending on a disturbance.

REFERENCE SIGNS LIST

11, 12, 13 metal sheet

14 electrode

15 spacer

16 existing weld

1. A resistance spot welding method of squeezing, by a pair ofelectrodes, parts to be welded which are a plurality of overlappingmetal sheets and passing a current while applying an electrode force tojoin the parts to be welded, wherein main current passage includes twoor more electrode force application steps including a first electrodeforce application step and a second electrode force application stepfollowing the first electrode force application step, and an electrodeforce F₁ in the first electrode force application step and an electrodeforce F₂ in the second electrode force application step in the maincurrent passage satisfy a relationship F₁>F_(2,) and an electrode forceswitching point T_(f) from the first electrode force application step tothe second electrode force application step in the main current passageis set to satisfy the following Formulas (1) to (3): in the case whereT_(A)≤0.8×T₀,T _(A) ≤T _(f) <T ₀  (1) in the case where 0.8×T₀<T_(A)≤T₀or0.9×R₀≤R_(A)≤R₀,0.9×T ₀ <T _(f)<1.1×T ₀  (2) in the case where R_(A)<0.9×R₀,T ₀ <T _(f) ≤T ₀+2×(R ₀−R_(A))/R ₀ ×T _(m)  (3) where T₀ is a referenceelectrode force switching point from the first electrode forceapplication step to the second electrode force application step, T_(m)is a total welding time in the main current passage, R_(A) is a timeintegration value of a resistance between the electrodes from currentpassage start of the main current passage to the reference electrodeforce switching point T₀, R₀ is a time integration value of a resistancebetween the electrodes from current passage start to the referenceelectrode force switching point T₀ in the case where current passage isperformed under a same condition as the main current passage when theparts to be welded have no disturbance, and T_(A) is a time at which atime integration value of a resistance between the electrodes in themain current passage reaches R₀.
 2. The resistance spot welding methodaccording to claim 1, wherein the reference electrode force switchingpoint T₀ satisfies the following formula:0.1×T _(m) ≤T ₀≤0.8×T_(m).
 3. The resistance spot welding methodaccording to claim 1, comprising: performing test welding; andperforming actual welding including the main current passage, after thetest welding, wherein in main current passage in the test welding, atime variation curve of an instantaneous amount of heat generated perunit volume and a cumulative amount of heat generated per unit volumethat are calculated from an electrical property between the electrodesin forming an appropriate nugget by performing current passage byconstant current control are stored, and in the main current passage inthe actual welding, the time variation curve of the instantaneous amountof heat generated per unit volume and the cumulative amount of heatgenerated per unit volume that are stored in the main current passage inthe test welding are set as a target, and a current passage amount iscontrolled according to the target.
 4. A weld member production methodcomprising joining a plurality of overlapping metal sheets by theresistance spot welding method according to claim
 1. 5. The resistancespot welding method according to claim 2, comprising: performing testwelding; and performing actual welding including the main currentpassage, after the test welding, wherein in main current passage in thetest welding, a time variation curve of an instantaneous amount of heatgenerated per unit volume and a cumulative amount of heat generated perunit volume that are calculated from an electrical property between theelectrodes in forming an appropriate nugget by performing currentpassage by constant current control are stored, and in the main currentpassage in the actual welding, the time variation curve of theinstantaneous amount of heat generated per unit volume and thecumulative amount of heat generated per unit volume that are stored inthe main current passage in the test welding are set as a target, and acurrent passage amount is controlled according to the target.
 6. A weldmember production method comprising joining a plurality of overlappingmetal sheets by the resistance spot welding method according to claim 2.7. A weld member production method comprising joining a plurality ofoverlapping metal sheets by the resistance spot welding method accordingto claim
 3. 8. A weld member production method comprising joining aplurality of overlapping metal sheets by the resistance spot weldingmethod according to claim 5.